The GOG version starts with the quake4key file with the first line empty, then after first launch a key is written to the file. Cause I am pretty sure "DRM-Freed" version of Quake 4 on GOG does not generate a valid CD key. If GOG does not provide CD key as "Serial Keys" as they do with Neverwinter Nights Diamond, there is no way to run GOG version natively on Linux. But, as I checked SteamDB page of the game, Steam does provide quake4key in q4base\quake4key but unlike some other Steam games with third party CD keys, Steam client does not show the CD key the game probably requires to be launched via Steam to generate the key.
Rea987: As I do not own GOG version of the game, I do not know about the situation there. As I do not own GOG version of the game, I do not know about the situation there. Copying non-encrypted key to ~/.quake4/q4base works fine. \Program Files\id Software\Quake 4\q4base\quake4key as a " non-encrypted" key. However, Windows version of the game asks for the key during the installation and saves it to. The key that you entered is saved to ~/.quake4/q4base/quake4key as an encrypted key. As I have just tried with Quake 4 retail disk, native Linux version of the game asks CD key during the first launch.